Very high pressure discharge lamps include an arc tube containing an inert gas, mercury vapor, and two electrodes positioned at opposite ends of the arc tube. An arc discharge is established in the arc tube by supplying an electrical current to the electrodes. The very high pressure discharge lamp is typically utilized for projection applications, where the optical system requires point-like light sources. To achieve such optical performance, the arc length must be on the order of 1.0-1.5 millimeters. The lamps typically include an arc tube constructed of heat resistant and optical transparent material such as quartz, tungsten electrodes, mercury vapor and an inert starting gas. The electrodes are constructed of a tungsten rod with a tungsten coil attached to one end.
Typically, the electrode tip in very high pressure discharge lamps reaches temperatures close to or even above the melting point of tungsten. This is necessary to prevent movement of the point of arc attachment to the electrode, also called the arc root. However, if the electrodes become too cold, the molten tip solidifies and collapses to a relatively flat surface. The arc attachment becomes unstable, leading to sudden arc movement or jumping. Moreover, the distance between electrodes increases, thereby reducing the performance of the light collecting optics. On the other hand, if the electrodes become too hot, the molten region increases, leading to a meltback condition where the distance between electrodes increases, thereby reducing the performance of the light collecting optics. Moreover, during the meltback, increased amounts of tungsten are evaporated from the electrodes and deposited on the arc tube walls, leading to poor lamp maintenance. Therefore, the operation of very high pressure discharge lamps needs to be optimized in order to achieve the most beneficial electrode temperature.
A problem in establishing the optimal temperature for the two electrodes of a discharge lamp may arise when the lamp current is varied. Such a situation occurs as the lamp voltage increases. To maintain constant power conditions, prior arc ballast circuitry reduces the lamp current. As a result, the tip temperature of one electrode is reduced such that the tip starts to solidify. Upon solidification, the tip of the electrode diminishes, leading to a larger arc gap and therefore, a larger lamp voltage. The current supply to the lamp decreases and the electrode tip temperature is further reduced. The process continues until the electrode surface becomes flat. Arc instability may then occur.
Furthermore, a very high pressure discharge lamp is usually mounted in a reflector, which changes the thermal environment of the arc tube. One end of the lamp, and thus one electrode, may be hotter than the other end. In typical applications, very high pressure discharge lamps are operated with forced air cooling, which is usually directed to both sides of the lamp or to the upper side of the lamp. Depending on the configuration of the cooling airflow, different lamp performance is achieved.
During the lifetime of a discharge lamp, the structure of the electrodes may change due to tungsten transport from the tip of the electrode. In cases where one of the electrodes has started to melt back, its ability to conduct heat from the electrode tip changes, and the flattening process may accelerate, leading to early lamp failure. Using preshaped electrodes cannot compensate for most of these asymmetries, because they are unpredictable. Different electrode shapes require additional devices to permit proper mounting of the lamp in the system in which it is employed.
Techniques and circuits for operating high pressure discharge lamps are disclosed in U.S. Pat. No. 5,608,294, issued Mar. 4, 1997 to Derra et al.; U.S. Pat. No. 6,232,725, issued May 15, 2001 to Derra et al.; U.S. Pat. No. 6,239,556, issued May 29, 2001 to Derra et al.; and International Publication No. WO 2004/002200, published Dec. 31, 2003. All of the known prior art techniques have had one or more drawbacks and disadvantages.
Accordingly, there is a need for methods and apparatus for operating very high pressure discharge lamps with improved performance and lifetime.